// What the richeditor's TipTap schema can show (UI-SPEC RichEditorField,
// "Lossy note"). Stored HTML with any other element still opens: the field
// keeps the stored string until the administrator edits (the untouched
// rule), and the HTML view edits it without loss. This module has no TipTap
// import, so the field can decide before the editor chunk loads.

/** Elements the editor shows; compared by lower-cased tag name. */
export const EDITOR_TAGS: ReadonlySet<string> = new Set([
  'p',
  'br',
  'strong',
  'b',
  'em',
  'i',
  'u',
  's',
  'h3',
  'ul',
  'ol',
  'li',
  'blockquote',
  'a',
  'div',
  'span',
])

/**
 * Parses html into an inert template: nothing in it runs or loads (no
 * scripts, no image requests), so stored markup can be inspected safely.
 */
export function inertFragment(html: string): DocumentFragment {
  const template = document.createElement('template')
  template.innerHTML = html
  return template.content
}

/** The distinct lower-cased tag names of html's elements, in document order. */
export function tagNames(html: string): string[] {
  if (html === '') {
    return []
  }
  const out = new Set<string>()
  for (const element of Array.from(inertFragment(html).querySelectorAll('*'))) {
    out.add(element.localName.toLowerCase())
  }
  return [...out]
}

/** Whether html holds an element the editor cannot show (the lossy note). */
export function isLossy(html: string): boolean {
  return tagNames(html).some((name) => !EDITOR_TAGS.has(name))
}